Search for New Physics with Two Photons in the Final State with the ATLAS Detector

This thesis reports on the search for new physics in the diphoton decay channel with the proton-proton collision data collected by ATLAS at a centre-of-mass energy of $\sqrt{s}=8$~TeV in 2012 and $\sqrt{s}=13$~TeV in 2015 and 2016. A feasibility study of the search for a pseudoscalar $A$ decaying to a $Z$ boson and a 125~GeV Higgs boson in the context of an extended Higgs sector, namedly the two-Higgs-doublet models, is presented. The search is performed with a final state of two jets and two photons using 20.3~${\rm fb}^{-1}$ of data at $\sqrt{s}=8$~TeV. The expected sensitivity is found to be competitive with the analysis with a final state of two electrons or muons and two $\tau$ leptons, but less sensitive to the other searches with the Higgs decaying to a pair of $b$-quarks. Search for high mass resonances decaying to two photons at $\sqrt{s}=13$~TeV is also presented. The analysed dataset corresponds to an integrated luminosity of $3.2~{\rm fb}^{-1}$ in 2015 and $12.2~{\rm fb}^{-1}$ in 2016. Two searches are performed: one optimised for a generic spin-0 particle, and the other for a spin-2 particle using Randall-Sundrum graviton model as benchmark. Particular attention is paid to the understanding and the parameterisation of the line shape of a large width resonance. In the search using the collision data recorded in 2015, a moderate excess was observed at a diphoton invariant mass around 750~GeV with a best-fit width of around 45~GeV, corresponding to a local significances of 3.8-3.9 standard deviations, depending on the spin of the resonance being searched for. The global significances are estimated to be 2.1 standard deviations. The search is updated with 2016 data and no significant excess is observed. Limits on the production cross-section times branching ratio to two photons for the two resonance types are reported: for a spin-0 resonance, the fiducial cross-section ranges from 12~fb (51~fb) at 200~GeV to 0.2~fb (0.3~fb) at 2400~GeV, with the narrow (10\% of the mass) width hypothesis. A part of this document is also devoted to the study of photons that convert into electron-positron pairs before entering the calorimeter using their electromagnetic shower information. The conversion study has two objectives: improving the identification of photon conversion at high pile-up rate and providing an orthogonal classification method to assess the performance of the standard track-based algorithm.